Evaporator coils freeze up every 3-4 weeks and refrigerator temp becomes too high. Top 2/3 of evap coils freeze and along the right side near the fridge air return. I suspect there is a defrost system failure. Measured resistance across the heater and thermostat circuit which resulted in 21.7 ohms. The thermostat does not appear to open at room temp. What temp should it open? Either way, I don't think its the issue as it is showing to be closed with no resistance (0 ohms). I am suspecting the control board to be the issue. Is there a way to power to the defrost heater circuit manually to verify its operation? Is there a way to be sure it is the main control board? Do the freezer thermistors need to be replaced along with the board? Everything else seems to work just fine once I defrost it for about 3-4 weeks until the airflow stops because of ice.